This invention relates to plastic film sealing apparatus, and more particularly to a method and apparatus for effecting a thermal union among layers of heat sealable plastic film.
One common method for joining layers of heat sealable plastic films involves bringing a heated sealing bar into engagement with the plastic film layers. The heated bar melts the plastic and the pressure of the bar against the melted plastic forms a thermal weld at that point. When the proper bar temperature, sealing pressure, and weld time are employed, a good seal between the layers of the heat sealable plastic film is produced. Where the above parameters of temperature, pressure, and time can be standardized, this packaging method has proven especially useful in mass production applications, where the films may be used for packaging any of a wide variety of products.
The use of heat sealable plastic films in less standardized applications, however, has been somewhat retarded due to the relatively close tolerances which must be maintained with the above three parameters. Truly practical machines for small commercial and/or domestic users have not been feasible due in part to the great difficulties and expense, in mass production, of trying to adjust the heater thermostats closely enough to ensure consistently good results. Another limitation, for example in food store applications, has arisen where different types and thicknesses of plastic films are regularly encountered, where different numbers of layers of film are to be joined, where sealing times, pressures, and temperatures must therefore vary, where heated bar temperature stability and temperature uniformity are accordingly difficult to maintain, and so on.
In cases such as these, heat sealable plastic films have not provided the same convenience and economies found in the larger scale, more standardized operation. If the bar is too hot the films sag or melt through, follow the bar down as it is retracted, thus extending, thinning, and breaking into holes, or stick to the bar. If the bar is too cold there is no seal. If the temperature is non-uniform the seal is unacceptable. In commercial applications, the frequent trial runs and stabilization delays required for every change in the sealing operation too often outweigh the benefits afforded.